The present invention relates to a method of producing fuel from tall-oil soap by treating the soap with carbon dioxide. Tall-oil soap is a by-product of sulphate pulp manufacturing processes. The soap can be separated out from black liquor when concentrating the liquor into heavy black liquor. The tall-oil soap obtained has a pH of about 12 and is normally acidified with sulphuric acid to convert the soap to tall oil. A process of this kind is, in principle, carried out in all sulphate pulp mills. The tall oil obtained can be used as a crude product in the preparation of different chemicals or as fuel.
One drawback with this process is that relatively large volumes of sulphuric acid are required. The delivery of sulphur to the system makes it difficult to achieve a closed process in the mill while avoiding environmentally harmful emissions.
It has been proposed to treat tall-oil soap with carbon dioxide, with the intention of eliminating the use of sulphuric acid or reducing the amounts in which it is used. The proposed process uses inorganic carbonic acid which is generated by the reversible reaction between carbon dioxide and water: EQU CO.sub.2 +H.sub.2 O.revreaction.H.sub.2 CO.sub.3
The resultant carbonic acid reacts with the tall-oil soap in accordance with the following reactions: EQU RCOONa+H.sub.2 CO.sub.3 .revreaction.RCOOH+NaHCO.sub.3
However, only about half of the soap is converted to tall oil in these reactions.
Because the earlier processes that use carbon dioxide have been intended mainly to produce the purest possible tall oil, so as to enable the oil to be used in the preparation of different chemicals, the carbon dioxide treatment stage is followed by a sulphuric acid treatment stage. Although this process results in a reduction of the sulphuric acid otherwise required, it does not eliminate the use of sulphuric acid completely. An alternative solution proposes the onerous repetition of the carbon dioxide treatment in several treatment steps.
The present invention is based on the realization that tall-oil soap can be treated with carbon dioxide to obtain a fuel which contains tall oil that does not require the same degree of purity as tall oil intended for some other use. The inventive process affords the important advantage of enabling tall-oil soap to be used as fuel without the addition of sulphur.